1. Field of the Invention
The invention relates to an age-hardening copper alloy as material for producing casting molds.
2. Description of Related Art
The worldwide aim, especially of the steel industry, to pour semifinished product as close to final dimensions as possible, in order to save hot and/or cold working steps, has led since about 1980 to a series of developments, such as single roll and two-roll continuous casting methods.
In these casting methods, very high surface temperatures appear at the water-cooled cylinders or rolls during casting of steel alloys, nickel, copper, as well as alloys that are only rolled with difficulty in the pouring range of the melt. In the case of close to final dimension casting of a steel alloy, the temperatures are about 350° C. to 450° C., the continuous casting and rolling sleeves being made of a CuCrZr material having an electrical conductivity of 48 Sm/mm2 and a heat conductivity of about 320 W/mK. Materials based on CuCrZr were used up to now predominantly for continuous casting dies and casting wheels that were thermally highly stressed. In the case of these materials, the surface temperature drops cyclically to about 150° C. to 200° C., by the cooling of the casting rolls, with each revolution, shortly before the casting range. On the cooled rear side of the casting rolls, however, the temperature remains largely constant during the cycle, at about 30° C. to 40° C. The temperature gradient between the surface and the rear side in combination with the cyclical change in the surface temperature of the continuous casting rolls causes thermal stress in the surface region of the sleeve material.
According to investigations of the fatigue properties of the CuCrZr materials used up to now, at various temperatures, using an expansion amplitude of +/−0.3% and a frequency of 0.5 Hertz—these parameters approximately correspond to a rotational speed of the continuous casting rolls of 30 rpm—one may expect, for example, in the favorable case, a service life of 3000 cycles until cracks form, using a maximum surface temperature of 400° C., corresponding to a wall thickness of 25 mm above the water cooling. Therefore, the continuous casting rolls have to be reconditioned after as relatively early an operating time as about 100 minutes, for the purpose of removing surface cracks. In this context, the service life between reworking is, among other things, substantially dependent on the effectiveness of the lubrication/release agents at the casting surface, the constructive and process-conditioned cooling as well as the casting speed. For the purpose of exchanging the continuous casting rolls, the casting installation has to be stopped and the casting process has to be interrupted.
An additional disadvantage of the proven die material CuCrZr is its relatively low hardness of about 110 HBW to 130 HBW. However, in a single or two-roll continuous casting method it is not to be avoided that, even before the casting range, splashes appear on the roll surfaces. The solidified steel particles are then pressed into the relatively soft surfaces of the continuous casting rolls, whereby the surface quality of the cast strip of about 1.5 mm to 4 mm thickness are considerably impaired.
Compared to a CuCrZr alloy, the lower electrical conductivity of a known CuNiBe alloy, having an addition of up to 1% niobium, also leads to a higher surface temperature. Since the electrical conductivity behaves approximately proportionally to the heat conductivity, the surface temperature in the sleeve, of a continuous casting roll, made of the CuNiBe alloy as compared to a continuous casting roll having a sleeve made of CuCrZr, at a maximum temperature of 400° C. at the surface and 30° C. on the rear side will be increased to about 540° C.
Ternary CuNiBe and CuCoBe alloys do indeed basically demonstrate a Brinell hardness of more than 200 HBW, however the electrical conductivity of the standard semifinished products made of these materials, such as rod for manufacturing resistance welding electrodes or sheet or strip for manufacturing springs or leadframes, reach values of at most in the range of 26 Sm/mm2 to about 32 Sm/mm2. Under optimum conditions, with the use of these standard materials, a surface temperature of only about 585° C. could be reached at the sleeve of a continuous casting roll.
Even from the CuCoBeZr and CuNiBeZr alloys basically known from U.S. Pat. No. 4,179,314, no hints are seen that conductivity values of >38 Sm/mm2 in conjunction with a minimum hardness of 200 HBW could be achieved.
Within the scope of EP 0 548 636 B1, the use of an age-hardening copper alloy is also related art, which has 1.0% to 2.6% nickel that may be fully or partially replaced by cobalt, 0.1% to 0.45% beryllium, optionally 0.05% to 0.25% zirconium and possibly up to a maximum of 0.15% of at least one of the group of elements including niobium, tantalum, vanadium, titanium, chromium, cerium and hafnium, the rest being copper inclusive of production contaminations and the usual processing additives, having a Brinell hardness of at least 200 HBW and an electrical conductivity greater than 38 Sm/mm2 as the material for producing continuous casting rolls and wheels.
Alloys having these compositions, such as the alloys CuCo2Be0.5 or CuNi2Be0.5, have disadvantages in their hot forming capability, because of their relatively high alloying element content. However, high heat deformation strains are required to attain a fine grained product having a grain size <1.5 mm (as in ASTM E 112), starting from a coarse-grained cast structure having a grain size of several millimeters. In particular, for large format casting rolls, up to this point, sufficiently large continuous casting rolls have been producible only at very high expenditure; however, technical shaping devices are hardly available for realizing, at a justifiable cost, a sufficiently high hot kneading for recrystallization of the cast structure into a fine grain structure.